1. Field of the Invention
Generally, the present invention relates to the field of hearing aids and a method of fitting a hearing aid. The invention more specifically related to a system for estimating otherwise unknown transfer functions for an individual hearing aid. Moreover, the present invention relates to a method and system for adjusting or fitting of a hearing aid using an estimated vent parameter and more particularly to a computer implemented method and a computer system for fitting a hearing aid gain by estimating the best fit acoustic model of the hearing aid by modelling a performed measurement with transmission line theory.
2. Description of the Related Art
WO 03/034784 A1 describes a digital hearing aid system is described where a part of the system is intended for delivering sound into an ear canal of a hearing aid user and this part includes a vent or ventilation canal in order to reduce the occurrence of the known occlusion effect which is often experienced uncomfortable by the hearing aid user.
The geometry of individual ear canals of a hearing aid user interacts with the dimensions of the ventilation canal in determining the acoustic properties and hence the actual gain of the hearing aid.
Even if a hearing aid with a sealed plug is used, because of the individual ear canal geometry a leakage between the ear canal walls and the ear plug of the hearing aid may occur that influences the acoustic properties of the hearing aid. Such a leakage may even occur by using custom-made ear plugs or a hearing aid with a flexible ear plug, for example made by silicon, which normally adapts to the individual ear canal geometry of the user.
The fitting of a hearing aid is normally done by an audiologist in a fitting session in which the hearing threshold levels in certain frequency bands of the future hearing aid user is measured to determine the appropriate hearing aid gain over a frequency range. The frequency dependent measurement of the hearing loss or the so-called hearing threshold level (HTL) may be done by recording an audiogram. An audiogram is the graphical representation of a hearing test. It shows for each ear the minimum sound level required for the future hearing aid user to be able to hear sound per different frequency. The provided sound in the test may be produced by loudspeakers or a hearing aid like device which then also may measure the sound pressure at the eardrum at the hearing threshold.
The necessary gain to be provided by the hearing aid is then calculated based on the audiogram and further fitting rules. However, a leakage or even a ventilation canal (vent) present when using the actual hearing aid influences the sound pressure or other acoustical properties in the ear, and thus the actual gain of the hearing aid may not be properly taken into account in the calculation of the hearing aid gain. Hence, it may be a problem in the state of the art of hearing aid fitting routines that the hearing aid gain is not calculated based on the acoustic properties of the individual ear canal of the user with the actual hearing aid placed in the ear.
Thus, there is a need for improved techniques for fitting a hearing aid taking the acoustic properties of the hearing aid in the individual ear canal of the hearing impaired person into account.